1. Field of the Invention
The present invention relates generally to the field of downhole sampling and in particular to the downhole and onsite surface ultrahigh resolution spectroscopy using a tunable diode laser for measurement and estimation of parameters of interest of hydrocarbon samples prior to, during, or after capture in a sample chamber.
2. Summary of the Related Art
Typically, sample tanks are transported to laboratories for analysis to determination formation fluid properties based on the sample. The samples have to be transferred to a transportation tank and, in so doing, risk sample damage and spoilage due to pressure loss and formation of bubbles or asphaltene precipitation in the sample. Moreover, even if the sample is transferred successfully, it typically takes weeks or months to receive the laboratory analysis. Thus, there is a need for a quicker sample analysis for downhole in situ analysis and for onsite surface analysis.
Currently spectral analysis downhole or onsite using optical filters is limited to around 11 nm full width half maximum wavelength resolution. These filters are not suitable to distinguish between close peaks, resolve small peaks superimposed on larger peaks, or to identity differences in isotopes whose features are much smaller than 11 nm. Thus, there is a need for an analysis technique suitable for downhole and onsite surface spectrographic analysis of hydrocarbon samples with high resolution on the order of 1–4 nm or much better. Manufacturers of tunable diode lasers often claim 0.01 nm or better resolution.
The present invention addresses the shortcomings of the related art described above. The present invention provides an apparatus and method for onsite surface and downhole spectral analysis of formation fluid samples, whether filtrate, hydrocarbon related or brine samples collected downhole in an earth boring or well bore. The present invention provides a tunable diode laser (TDL) for ultrahigh resolution spectroscopy (UHRS) and corresponding estimation of parameters of interest of such samples based on correlation to these UHRS measurements. A sorption cooling apparatus is also provided to cool the TDL and UHRS downhole if necessary. A plurality of TDLs are ganged together in one embodiment to span a hydrocarbon band of frequencies from approximately 1650 nm to 1850 nm or to provide a tunable spectral range over selected areas of interest in selected frequency bands. In an embodiment a TDL for UHRS is provided downhole for real time UHRS measurements and estimation of parameters of interest from the UHRS measurements. In another embodiment the TDL UHRS is performed at the surface onsite or by the UHRS in the tool or via a separate UHRS system attached at the surface. The TDL UHRS of the present invention is also useful for analysis of gases and fluids and isotopes thereof while flowing in distribution pipelines to determine the purity, grade and identity of hydrocarbon bearing fluids and gases.